The present invention relates to a method of developing an electrostatic latent image on the surface of a photoconductive drum with a magnetic developer attracted on the surface of a developing roll disposed opposite to the photoconductive drum and made of a cylindrical permanent magnet having on its surface a plurality of magnetic poles circumferentially aligning with regular inter-pole space. More specifically, the present invention relates to a developing method capable of producing a high-quality printed image by minimizing uneven image density occurring along the moving direction of photoconductive drum.
In an electrophotographic or electrostatic imaging process, an electrostatic latent image on a photoconductive or dielectric surface of an image-bearing member is developed by bringing a magnetic brush of a magnetic developer on a developing roll into contact with the latent image. Then, the developed toner image is fixed directly or after transferred onto a recording sheet such as plain paper to give a final image.
The developing roll comprises a non-magnetic sleeve for attractively retaining thereon a developer and a permanent magnet disposed inside the sleeve and having on the surface thereof a plurality of magnetic poles. The sleeve is oppositely disposed to an image-bearing member with a certain distance so as to define a developing zone between the circumferential surfaces of the sleeve and the image-bearing member. The magnetic developer retained on the sleeve surface is transported to the developing zone by the relative rotation of the sleeve and the permanent magnet, and a toner in the magnetic developer is attracted to the latent image in the developing zone to produce toner image.
To meet the recently increasing requirement to develop low-cost and small-sized electrophotographic imaging machines represented by a copying machine, printer, etc., several proposals have been made on modifying the construction or changing the design of the developing roll. For example, a developing roll with no sleeve has been proposed to attractively retain magnetic developer on the permanent magnet surface directly and transport the retained magnetic developer to the developing zone by the rotation of the permanent magnet only (JP-A-62-201463).
The magnetic developer directly attracted on the permanent magnet surface forms undulated layer having the thickest portion on magnetic poles and the thinnest portion between neighboring poles. Therefore, in magnetic brush development using such a developing roll with no sleeve, a latent image is alternatively brushed with magnetic brushes in the thickest portion and the thinnest portion. Since there is a considerable difference in developability between the magnetic brushes in the thickest portion and the thinnest portion, uneven image density along the moving direction of the image-bearing member occurs in developed images, and in particular, the reproduction of half tone is unfavorably deteriorated. Solutions hitherto proposed for avoiding such uneven image density may include high-speed rotation of the permanent magnet, however, this is not practical because of increased driving torque, scattering of developer, generation of loud noise, etc.